I know exactly what you mean. The most DIFFICULT part is getting out there for the first time.
For me I sometimes trick myself into it.
Baby steps is the key. I just say to myself okay, just put on your running gear. You don't have to go. Just put it on. Then I tell myself to just do a warm up and see how I feel. I can always back out. Then I tell myself to just get out and do 10 minutes.....better than nothing at all, right?
Before I know it I'm jogging outside and am into it. By the time I've come home I feel sooooo much better and am ALWAYS glad I went. This reminds me how good it feels and it's easier to go out again next time. Might seem strange but it works for me.
First step is to force myself to change into my gear. The rest then follows. It does require some will power to force yourself to do that even when you don't want to sometimes though.
Remember how good you'll feel afterwards.
Hope it helps
Glen